This invention relates to a switching power supply circuit provided as a power supply in various electronic apparatus.
In recent years, thanks to the development of a switching element which can withstand comparatively high current and voltage of a high frequency, most of power supply circuits which rectify an ac voltage from a commercial power supply to obtain a desired dc voltage are formed as power supply circuits of the switching system.
A switching power supply circuit uses a high switching frequency to decrease the size of a transformer and other parts and is formed as a DC-DC converter of high power for use as a power supply for various electronic apparatus.
Incidentally, it is conventionally known that, if an ac input voltage is rectified, then the current flowing through a smoothing circuit has a distorted waveform, which deteriorates the power factor representative of efficiency in utilization of a power supply.
Further, higher harmonics originating from current of a distorted waveform have the possibility of having a bad influence on the load side, and therefore, a countermeasure for suppressing such distortion in a current waveform is required.
Thus, a switching power supply circuit is conventionally known wherein a power choke coil is inserted in series in a commercial ac power supply line to expand the conduction angle of the ac input current to achieve improvement of the power factor (so-called, choke input system).
FIG. 30 shows a configuration of a conventional switching power supply circuit which includes a countermeasure for improving the power factor according to the choke input system.
The power supply circuit shown in FIG. 30 adopts a combination of a current resonance converter of the separately excited type and a partial voltage resonance circuit as a configuration of the primary side.
Referring to FIG. 30, the power supply circuit shown includes a noise filter formed from a pair of filter capacitors CL and a common mode choke coil CMC for the line of a commercial ac power supply AC.
At the following stage of the noise filter, a full-wave rectification smoothing circuit is provided which includes a bridge rectification circuit Di and a smoothing capacitor Ci. A rectified smoothed voltage Ei (dc input voltage) is obtained across the smoothing capacitor Ci by cooperative full-wave rectification operation by the bridge rectification circuit Di and the smoothing capacitor Ci. The rectified smoothed voltage Ei has a level equal to the ac input voltage VAC.
Further, a power choke coil PCH is inserted in series between the noise filter and the bridge rectification circuit Di as seen in FIG. 30 in a line of the commercial ac power supply AC.
The current resonance converter which receives the dc input voltage to perform a switching operation includes two switching elements Q1, Q2 each in the form of a MOS-FET connected in half-bridge connection. Damper diodes DD1, DD2 each in the form of a body diode are connected in parallel in directions shown in FIG. 30 between the drains and the sources of the switching elements Q1, Q2, respectively.
A partial resonance capacitor Cp is connected in parallel between the drain and the source of the switching element Q2. The capacitance of the partial resonance capacitor Cp and the leakage inductance L1 of a primary winding N1 form a parallel resonance circuit (partial voltage resonance circuit). Thus, a partial voltage resonance operation wherein voltage resonance is exhibited only upon turning off of the switching elements Q1, Q2 is obtained.
In the power supply circuit, in order to drive the switching elements Q1, Q2 for switching, an oscillation and drive circuit 2 is provided which may be formed typically from an IC for universal use. The oscillation and drive circuit 2 includes an oscillation circuit and a drive circuit not shown. The oscillation circuit and the drive circuit cooperatively generate a drive signal (gate voltage) of a required frequency to be applied to the gates of the switching elements Q1, Q2. Consequently, the switching elements Q1, Q2 perform switching operation wherein they alternately switch on/off in a required switching frequency.
An isolating converter transformer PIT transmits a switching output of the switching elements Q1, Q2 to the secondary side. The primary winding N1 of the isolating converter transformer PIT is connected at one end thereof to a node (switching output point) between the source of the switching element Q1 and the drain of the switching element Q2 through a series connection of a primary side series resonance capacitor C1 so that the switching output is transmitted.
The primary winding N1 is connected at the other end thereof to the primary side ground.
The capacitance of the series resonance capacitor C1 and the leakage inductance L1 of the isolating converter transformer PIT including the primary winding N1 form a primary side series resonance circuit for achieving operation of the current resonance type as operation of the primary side switching converter.
Thus, from the foregoing description, the primary side switching converter described above provides operation of the current resonance type by the primary side series resonance circuit (L1-C1) and partial voltage resonance operation by the partial voltage resonance circuit (Cp//L1) described hereinabove.
In other words, the power supply circuit shown in FIG. 30 has a configuration which includes a combination of a resonance circuit for forming a primary side switching converter as that of the resonance type with another resonance circuit. In the present specification, a switching converter of the type just described is referred to as composite resonance converter.
Though not shown in the drawings, the isolating converter transformer PIT includes an EE type core which includes a combination of E type cores typically made of a ferrite material. A wiring receiving portion of the isolating converter transformer PIT is divided into winding receiving portions for the primary side and the secondary side, and the primary winding N1 and a secondary winding (N2A and N2B) described below are wound on a central magnetic leg of the EE type core.
A gap G is formed in the central magnetic leg of the EE type core. More particularly, the gap G is formed in a size of approximately 1.0 mm so that a coupling coefficient k of approximately 0.85 is obtained.
Further, in the circuit shown in FIG. 30, the numbers of turns of the secondary windings N2A, N2B and the primary winding N1 are set so that the induced voltage level per one turn (1 T) of the secondary side winding might be 5 V/T.
The secondary winding N2 of the isolating converter transformer PIT has a center tap and is therefore divided into two secondary windings N2A, N2B. An alternating voltage corresponding to a switching output transmitted to the primary winding N1 is excited in each of the secondary windings N2A, N2B.
The center tap of the secondary windings N2 is connected to the secondary side ground. A full-wave rectification circuit is connected to the secondary windings N2A, N2B and includes rectification diodes D01 D02 and a smoothing capacitor C0. Consequently, a secondary side dc output voltage E0 is obtained as a voltage across the smoothing capacitor C0. The secondary side dc output voltage E0 is supplied to a load not shown and is inputted also as a detection voltage for a control circuit 1 described below.
The control circuit 1 supplies a detection output corresponding to a level variation of the secondary side dc output voltage E0 to the oscillation and drive circuit 2. The oscillation and drive circuit 2 drives the switching elements Q1, Q2 with a switching frequency which varies in response to the detection output of the control circuit 1 inputted thereto. As the switching frequency of the switching elements Q1, Q2 is varied in this manner, the level of the secondary side dc output voltage is stabilized.
According to such a configuration for improvement of the power factor by the choke input system as shown in FIG. 30, the power choke coil PCH inserted in series in the line of the commercial ac power supply AC as described hereinabove smoothes the power in the frequency region of the commercial ac power supply and expands the conduction angle of the ac input current IAC to achieve improvement of the power factor.
It is to be noted that Japanese Patent Laid-open No. 2003-189617 discloses a related switching power supply circuit.
With the circuit of FIG. 30 which adopts the choke input system, however, reactive power which arises from iron loss of the core and copper loss of the coils is produced by the power choke coil PCH. The reactive power produced by the power choke coil PCH in this manner deteriorates the ac to dc power conversion efficiency of the power supply circuit.
If the inductance value of the power choke coil PCH is set to a higher value in order to obtain a sufficient effect of power factor improvement, then such iron loss and copper loss as mentioned above are likely to increase, which gives rise to further deterioration of the power conversion efficiency.